International Telecommunication Union sets Global Standard for Metro Networks Standard needed to satisfy the demands of voice, data and multimedia services for low-cost short-haul transport solutions in urban centres.

  • The ITU has set a global standard for Metro ‘Optical Fiber’ Networks that will expand the use of Coarse Wavelength Division Multiplexing (CWDM) in metropolitan networks.
  • This standard is necessary to meet the increasing demand of voice, data and multimedia services for low-cost short-haul optical transport solutions.
  • Where the distances are shorter and the need for capacity is less, CWDM applications are able to use wider channel spacing and less expensive equipment, yet achieve the same quality standards of long-haul optical fiber systems.

Figure-CWDM channel Spacing

Originally, the term “coarse wavelength division multiplexing” was fairly generic, and meant a number of different things. In general, these things shared the fact that the choice of channel spacings and frequency stability was such that erbium doped fiber amplifiers (EDFAs) could not be utilized. Prior to the relatively recent ITU standardization of the term, one common meaning for coarse WDM meant two (or possibly more) signals multiplexed onto a single fiber, where one signal was in the 1550 nm band, and the other in the 1310 nm band.

In 2002, the ITU standardized a channel spacing grid for use with CWDM (ITU-T G.694.2), using the wavelengths from 1270 nm through 1610 nm with a channel spacing of 20 nm. (G.694.2 was revised in 2003 to shift the actual channel centers by 1, so that strictly speaking the center wavelengths are 1271 to 1611 nm. [2]) Many CWDM wavelengths below 1470 nm are considered “unusable” on older G.652 specification fibers, due to the increased attenuation in the 1270-1470 nm bands. Newer fibers which conform to the G.652.C and G.652.D standards, such as Corning SMF-28e and Samsung Widepass nearly eliminate the “water peak” attenuation peak and allow for full operation of all 20 ITU CWDM channels in metropolitan networks.

The Ethernet LX-4 10 Gbit/s physical layer standard is an example of a CWDM system in which four wavelengths near 1310 nm, each carrying a 3.125 gigabit-per-second (Gbit/s) data stream, are used to carry 10 Gbit/s of aggregate data.

The main characteristic of the recent ITU CWDM standard is that the signals are not spaced appropriately for amplification by EDFAs. This, therefore, limits the total CWDM optical span to somewhere near 60 km for a 2.5 Gbit/s signal, which is suitable for use in metropolitan applications. The relaxed optical frequency stabilization requirements allow the associated costs of CWDM to approach those of non-WDM optical components.

CWDM is also being used in cable television networks, where different wavelengths are used for the downstream and upstream signals. In these systems, the wavelengths used are often widely separated, for example the downstream signal might be at 1310 nm while the upstream signal is at 1550 nm.

An interesting and relatively recent development relating coarse WDM is the creation of small form factor pluggable (SFP) transceivers utilizing standardized CWDM wavelengths. GBIC and SFP optics allow for something very close to a seamless upgrade in even legacy systems that support SFP interfaces. Thus, a legacy switch system can be easily “converted” to allow wavelength multiplexed transport over a fiber simply by judicious choice of transceiver wavelengths, combined with an inexpensive passive optical multiplexing device.

Passive CWDM is an implementation of CWDM that uses no electrical power. It separates the wavelengths using passive optical components such as bandpass filters and prisms. Many manufacturers are promoting passive CWDM to deploy fiber to the home.

A WDM-PON designer must decide on the appropriate wavelengths and their spacing, based on which the selection of devices may differ significantly. The two major wavelength options—coarse WDM (CWDM) PON and dense WDM (DWDM) PON. The OLT and ONUs have optical transmitters, receivers, multiplexers, and demultiplexers. Several WDM-PON transmitter options have been proposed. Receiver options, which are dependent on both loss and protocols, various multiplexers and demultiplexers to be deployed at remote nodes (RNs). Wavelength Options Wavelength spacing of more than 20 nm is generally called coarse WDM (CWDM). Optical interfaces, which have been standardized for CWDM, can be found in ITU G.695, while the spectral grid for CWDM is defined in ITU G.694.2. If the complete wavelength range of 1271 nm to 1611 nm, as defined in ITU G.694.2, is used with 20 nm spacing, then a total 18 CWDM channels are available. A low-water-peak fiber defined in ITU G.652 C & D, which eliminates power attenuation in the 1370–1410 nm range seen in a normal single-mode fiber, can be used for this wide spectrum of transmission. The dispersion parameter indicates signal broadening, and this factor may limit the transmission distance as the data rate becomes higher.

Since strict tuning of wavelengths is not needed for the CWDM-PON, a thermal control part, called a thermo- electric cooler (TEC), is not required, making it cheaper than the DWDM-PON. Furthermore, the wavelength multiplexer with low channel crosstalk can be implemented easily for CWDM. It has been argued that the total system cost is 40% cheaper for the CWDM-PON. The primary disadvantage of CWDM is that the number of channels is limited; therefore, the CWDM-PON lacks in scalability, especially when a normal single-mode fiber with water-peak attenuation range is used. Another disadvantage is that the shorter wavelength channels experience higher loss, thereby limiting the transmission distance or splitting ratio. A brief example of the CWDM-PON can be found in the so-called “triple-play” PON service, where the 1550 nm wavelength channel is used for optional video CATV, the 1490 nm wavelength channel is used for downstream voice and data, while the 131 nm wavelength channel is used for upstream transmission. An expanded application adopts 1360–1480 nm CWDM channels for premium business services, while usual triple-play services are provided to normal subscribers. DWDM has wavelength spacing that is far lesser than that of CWDM, typically less than 3.2 nm, because DWDM has been developed to transmit many wavelengths in a limited spectrum region where an EDFA can be used. A DWDM-PON is expected to be very useful for providing enough bandwidth to many subscribers, and it is regarded as the ultimate PON system. ITU G.692 defines a laser grid for point-to-point WDM systems based on 100 GHz wavelength spacing with a centre wavelength of 193.1 THz (1553.52 nm) over the frequency region of 196.1 THz (1528.77 nm) to 191.7 THz (1563.86 nm). This 100 GHz spacing has been applied to many DWDM systems. But, 50 GHz spaced laser diodes (LDs) and filters are commercially available today, and they can be used to increase the number of channels. Also, wavelengths reaching up to 1600 nm have been used to exploit the cyclic property of the AWG, by having just one AWG at a remote node for demultiplexing and multiplexing in downstream and upstream directions, respectively. In a DWDM-PON, the wavelength of each optical source and the center wavelength of the WDM filter should be monitored and controlled carefully to avoid crosstalk between adjacent channels. Therefore, the DWDM PON costs more than the CWDM-PON in field deployment since it needs wavelength-tuned devices and temperature control.

Coarse wavelength division multiplexing (CWDM) is set to introduce multi wavelength optical systems cheaply into the metro network. Compared to DWDM, CWDM achieves cost reduction via the use of cheaper wide channel spacing filters, which in turn allow the use of cheaper uncooled lasers in CWDM systems. However, because CWDM systems are non-amplified, the attainable system.

CWDM network design approach that can be employed to maximize the perimeter of a 16-channel CWDM transmission network based on a G.652D-class fiber [zero water peak fiber (ZWPF)], such as All Wave.

All 16 channels of the full spectrum (FS) CWDM channel plan extending from 1310* nm in the O-band to 1610 nm in the L-band can be supported on a single fiber, A wavelength assignment algorithm assigns wavelength bands to the nodes in such a way that the accumulated filter losses incurred by the O-band wavelengths around the ring are minimized and increase to its maximum in the L-band. In contrast, ZWPF attenuation as a function of wavelength reaches a maximum in the O-band and monotonically decreases to a minimum value in the L-band. Hence, the increasing attenuation slope of the filters toward longer wavelengths is partially offset by a falling attenuation slope of the fiber loss curve.

Therefore, the main purpose of this approach is to increase the application range of nonamplified CWDM for metro rings, to demonstrate the competitiveness of CWDM for new segments of the metro market that are dominated by short-reach DWDM, time division multiplexing (TDM), and space division multiplexing (SDM) for metro deployment. This will further enhance the competitiveness of G.652D fiber since these fibers offer a seamless support of all 16 CWDM channels on a single fiber compared to standard G.652 (SSMF), which only supports a maximum of 12 CWDM channels. The FS-CWDM channel plan being standardized by the ITU-T comprises 16 non-amplified channels, with center wavelengths starting at 1310 nm with 20-nm channel spacing is shown in Table-3.1


Table- IUT-Based CWDM Channel Plan for 16 CWDM Channels

SN Fiber att.[B/km] Laser Wavelength (nm) Channel# Band
1 0.36 1310 1  

O- Band

2 0.335 1330 2
3 0.322 1350 3  



4 0.311 1370 4
5 0.333 1390 5
6 0.291 1410 6
7 0.281 1430 7  








S-, C-, L-band

8 0.272 1450 8
9 0.266 1470 9
10 0.260 1490 10
11 0.254 1510 11
12 0.252 1530 12
13 0.250 1550 13
14 0.250 1570 14
15 0.257 1590 15
16 0.266 1610 16 quad play architecture

Figure: quad play architecture

The proposed quad play structure is not like traditional tripe play architecture. The Video signal and the ISP’s connection will combinely enter into a MUX and the modulated signal will enter into OLT. The OLT will propagate the modulated signal towards the ONTs. And as usual from the ONT’s ethernet port, the user will receive the signals. In traditional triple play, OLT did the modulation and OLT becomes busier and its uses was high, but in Quad play architecture the modulation was done in MUX, for that the OLT does not spent time for modulation.

Synchronous Digital Hierarchy (SDH)

 Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). At low transmission rates, data can also be transferred via an electrical interface. The method was developed to replace the plesiochronous digital hierarchy (PDH) system for transporting large amounts of telephone calls and data traffic over the same fiber without synchronization problems.

In digital telephone transmission, “synchronous” means the bits from one call are carried within one transmission frame. “Plesiochronous” means “almost (but not) synchronous,” or a call that must be extracted from more than one transmission frame.

SDH uses the following Synchronous Transport Modules (STM) and rates: STM-1 (155 megabits per second), STM-4 (622 Mbps), STM-16 (2.5 gigabits per second), and STM-64 (10 Gbps).

Today’s carrier backbone networks are supported by synchronous optical network (SONET) and synchronous digital hierarchy (SDH) transmission technologies. SONET is the standard used in the United States and SDH is the standard used outside the United States.

SONET/SDH specification outlines the frame format, multiplexing method, and synchronization method between the equipment, as well as the specifying optical interface.

SONET/SDH will continue to play a key role in the next generation of networks for many carriers. In the core network, the carriers offer services such as telephone, dedicated leased lines, and Internet protocol (IP) data, which are continuously transmitted.

Advantages of SDH Network

      Synchronous Digital Transmission

Until the introduction of SONET in the mid-1980s, plesiochronous digital hierarchy (PDH) systems commonly used data multiplexing technology. The primary problem with PDH was that to extract low-speed traffic, all traffic that was multiplexed to higher speeds had to be de-multiplexed into lower speeds. With PDH, the equipment had to support multiplexing and de-multiplexing the signal, adding cost and complexity to the network.

SONET was introduced as a synchronous transmission system that could directly extract low-speed signals from multiplexed high-speed traffic. Based on the ANSI standard, the CCITT approved the international standard known as SDH based on the SONET technology.

      Mid-Span Meet

The adoption and acceptance of SONET allowed carriers to be able to choose equipment from different vendors instead of using only a single vendor with a proprietary optical format. The ability to mix equipment from different vendors in one system is called the “Mid-Span Meet”.


SONET and SDH give carriers much more bandwidth to carry voice and data traffic than PDH technology. The base rate for SONET is 51 Mbps. Synchronous transport signal (STS-n) refers to the SONET signal in the electrical domain, and optical carrier (OC-n) refers to the SONET signal in the optical domain. The base rate for SDH is 155 Mbps. Synchronous transport module (STM-n) refers to the SDH signal level in both the electrical and optical domains. See Table

Table -: Synchronous Transport Signal (STS) & Synchronous Transport Module (STM)

North America STS Level North America OC Level European STM Level Line Rates (Mbps)
STS-1 OC-1 N/A 51.84
STS-3 OC-3 STM-1 155.52
STS-12 OC-12 STM-4 622.08
STS-48 OC-48 STM-16 2,488.32
STS-192 OC-192 STM-64 9,953.28
STS-768 OC-768 STM-256 39,813.12



Carriers require an extremely reliable network and cannot afford downtime. Therefore, most SONET/SDH networks have a ring structure, which adds high reliability to the overall transmission network. Even if the optical fiber is cut, the transmission path is backed-up and restored within 50 ms.

A SONET/SDH transmission network is composed of several pieces of equipment, including:

  1. Terminal multiplexer (TM)
  2. Add-drop multiplexer (ADM)
  3. Repeater
  4. Digital cross-connect system (DCS)

    Various Parameter Calculation of CWDM Technology

Crosstalk effect of CWDM technology calculated and some example link also discussed in this section.

   Fiber Loss and Filter Loss Calculation:

Summarizing the impact of loss originating from various sources. First, there is the spectrally dependent loss of the optical transmission fiber and the different channels of the CWDM multiplexer with a significant variation between the CWDM channels due to the wide transmission bandwidth of 300 nm. Secondly, there are other sources of loss such as connector loss that are generally independent of wavelengths or particular channel numbers.

   Fiber loss W( λi , k):


P=Source Power

S=receiver sensitivity

F(λi , k)= filter insertion loss incurred by wavelength assigned to  ONTk

C(λi , k)= connector loss incurred by wavelength assigned to ONTk

Filter loss F( λi , k):


ONTk -to-OLT path is the higher-loss-path:

The path ONTk -to-OLT, the filter loss F(λi, k) incurred by a wavelength λi is calculated as follows:

if n+1> 2k


f mux= mux loss at ONT Nk

f add = add loss at ONT Nk

f exp = express losses at nodes Nk +1 to Nn

f drop = drop loss at the Splitter

f demux = demux loss at the ONT.


OLT-to-ONTk path is the higher loss path:

On the path from the OLT-to-ONTk, the filter loss incurred by a wave¬length  , is:

F(λi , k)=  f mux  + f add + (k-1) f exp + f drop + f demux                      if 2k>n+1



f mux= mux loss at the OLT

f add = add loss at the OLT

f exp = express losses at ONTs N1  to Nk-1

f drop = drop loss at the Splitter

f demux = demux loss at the ONT.

Therefore, combining Equations, the filter loss incurred by a wavelength, assigned to ONTk is given by:


  1. c) Connector loss C( λi, k):

Following the same procedure as for the filter loss derivations, the connector loss incurred by a wavelength  λi  assigned to ONTk is given by:

where Csp is the connector loss per span, which is the same for each node and independent of wavelength. So for the estimated loss of a cable plant, calculate the approximate loss as:

Path loss (Source to Destination) =  (0.5 dB X # connectors) + (0.2 dB X# splices) + (fiber attenuation X the total length of cable).


The basic discussion between DWDM and CWDM has been discussed and a proposed quad play architecture using CWDM technology has been discussed. GPON quad play architecture is based on using MUX instead of using EDFA. Direct modulated laser also used.